When we STAND straight upright (fig. 26) the centre of gravity of the head is directly over a point midway between the two ankles; and the plane of gravity, represented by the vertical line in the figure, descends, from the head, through the spine, pelvis, and lower limbs, to the key-bone of the instep. And you observe that, between the head and the ankle, the skeleton is not quite straight, but is arranged in six curves, which are, alternately, in front of and behind the line of gravity. Of these curves the upper three are in the spine. They are well marked; the uppermost (_a_) is in the neck and is directed forwards; the next (_b_) is in the back and is directed backwards; the third (_c_) is in the loins and is directed forwards. The fourth curve (_d_), less distinct than those above it, is in the pelvis and is directed backwards. The fifth and sixth curves are very slight; the fifth (_e_), directed forwards, is at the hip-joint; and the sixth, (_f_), directed backwards, is at the knee. The last two curves, though slight, are not unimportant; and they contribute very much to our comfort and to prevent fatigue when we are standing: they do so in the following way. The strong ligaments of the hip are placed towards the _fore_ part of the joint, that is, in _front_ of the line of gravity; and the strong ligaments of the knee are placed towards the _back_ part of the joint, that is, _behind_ the line of gravity. It follows that when these joints are fully extended they are “locked,” as it is termed, just as is a hinge when opened to a little beyond the straight line; and, by this means, the muscles are set at rest, and we are able to maintain the erect posture, for some time, steadily and without fatigue. When standing upright in this way, at rest on both legs, or on one leg in the military position of “at ease,” and the muscles are off their guard, if a sudden and unexpected, though slight, pressure be made upon the ham, so as to bend the knee a little and throw the joint in front of the line of gravity, the man will drop, unless the muscles come quickly to the rescue--a tendency which has not escaped the observation of school-boys. In BENDING or BOWING (fig. 27) the head is carried forwards; and, to maintain the balance, the opposite pole of the trunk is carried backwards, so as to preserve the line of gravity still over the ankles.